No you didn't understand he's the owner of the website...lol I'm co-admin. So it's our site.^^ I know I said it a little complicated. It's his forums and his site and he paid for the domain and he's the leader and I'm just helping him out. So now it should be clear. I haven't known that he's registered here. I just saw him the day before yesterday and then he told me that I would need to subscribe to download things and that he already did and then I said that I don't have to subscribe since he already did so he can install it on our forums, the forums of him and me. So understood now?

Yeah that's the only thing I think you should add. Like a posting feature, each month a member would need X number of points or posts to be subscribed to the supporters group. Or it could even be per week.

Well yeah but you have to see that it's actually work and the finished plugins/themes etc. are actually a "product" and the most products cost money. You can even be happy that MyBB is & stays free.

Of course like my suggestion of making it able with 100 posts or more would be nice, too since you don't get 100 quality posts that fast and it would be nice and for those who can actually subscribe and don't want to post, they could subscripe then.
But you have to respect that decision of him. Like I said plugins are "products".